This invention pertains to diagnostic X-ray apparatus and, particularly, to means for limiting the X-ray entrance exposure rate for fluoroscopy.
During fluoroscopy of an anatomical region, the X-ray tube current is set low as compared with the current required for radiographing the same region and the exposure interval is usually quite long. Government regulations recommend that the X-ray exposure rate used in fluoroscopy should be as low as is consistent with the fluorscopic requirements and shall not exceed 10 Roentgens per minute (10R/min.), measured in air, at the position or plane where the X-ray beam enters the patient. The requirement applies to the use of X-ray intensifier equipment as well as conventional direct viewing fluoroscopic screens. In some X-ray apparatus, complying with these requirements results in the producticn of fluoroscopic images in which useful diagnostic information is obscured. In recently developed cardiovascular examination equipment, the patient is supported on an X-ray transmissive table top with the X-ray tube supported on one side and an image receptor, usually an electronic image intensifier, on the other side. In this equipment, the X-ray source or tube and intensifier are mounted for angulating jointly about the patient to obtain X-ray views of blood vessels at different perspectives. An example of such equipment is described in U.S. Pat. No. 4,339,825, dated July 13, 1982, and assigned to the assignee of the present application. This apparatus comprises an L-shaped arm (L-arm) that is rotatable about a vertical axis. A U-shaped arm (U-arm) is mounted to the upstanding leg of the L-arm for rotating or angulating about a horizontal axis. An X-ray image intensifier is mounted to the outboard end of one leg of the U-arm and an X-ray source, namely, an X-ray tube and collimator assembly is mounted to the outboard end of the other leg of the U-arm and the central ray emanating from the focal spot of the X-ray tube is directed toward the center of the image input plane of the image intensifier. A television camera is arranged for viewing the optical image produced by the image intensifier and for displaying the image on a television screen as is characteristic of modern fluoroscopy techniques. In this equipment, the X-ray tube is mounted for being moved toward and away from the patient who is supported in the beam path between the tube and the image receptor. Tube movement is made permissible so that more oblique angles of anatomical views can be obtained.
In L/U arm equipment the plane at which the X-ray beam enters the patient's body can, by way of example, be 30 cm. from the input plane of the image receptor. With the focal spot of the X-ray tube set at the usually desired minimum distance from the image intensifier input plane, it is possible to design the controls so that X-ray tube current will not be so great as to exceed the 10R/min. entrance dose rate. However where the flexibility of moving the X-ray tube toward and away from the patient is allowed, different conditions prevail. As the X-ray tube focal spot is moved closer to the patient, the permissible 10R/min. entrance dose can be exceeded and if it is moved farther from the standard minimum distance, entrance dose will decrease but so will the emergent X-ray exit dose. In the latter case, the reduced intensity of the emergent beam results in under-excitation of the image receptor and a consequent loss of diagnostic information in the fluoroscopic image.
In fluoroscopy, there are several variables that depend on the part of the anatomy or the density of the anatomy that is to be fluorographed. The peak kilovoltage (kVp) applied between the anode and cathode of the X-ray tube and the milliamperage (mA) or current flowing through the X-ray tube are selected on the basis of the density and thickness of the anatomical region of interest. As indicated already, the focal spot-to-image-distance (SID) also has a bearing on the intensity of the image produced in the receptor.